Six other amino acids are considered conditionally essential in the human diet, meaning their synthesis can be limited under special pathophysiological conditions, such as prematurity in the infant or individuals in severe catabolic distress.[2] These six are arginine, cysteine, glycine, glutamine, proline, and tyrosine. Six amino acids are non-essential (dispensable) in humans, meaning they can be synthesized in sufficient quantities in the body. These six are alanine, aspartic acid, asparagine, glutamic acid, serine[2], and selenocysteine (considered the 21st amino acid). Pyrrolysine (considered the 22nd amino acid) is not used by humans; thus, it is non‑essential.

Recommended daily intake

Estimating the daily requirement for the indispensable amino acids has proven to be difficult; these numbers have undergone considerable revision over the last 20 years. The following table lists the WHO and United States recommended daily amounts currently in use for essential amino acids in adult humans, together with their standard one-letter abbreviations.[6][7]

The recommended daily intakes for children aged three years and older is 10% to 20% higher than adult levels and those for infants can be as much as 150% higher in the first year of life. Cysteine (or sulfur-containing amino acids), tyrosine (or aromatic amino acids), and arginine are always required by infants and growing children.[6][8]

Although plants tend to have less protein per weight than animal sources such as eggs or milk, they are nevertheless “complete” in that, as a whole, they contain all of the amino acids essential in human nutrition.[9] The same is true for algae and marine phytoplankton. Eating various plant foods over an extended period of time can provide a protein of higher biological value.[10] Certain native combinations of foods, such as corn and beans, soybeans and rice, or red beans and rice, contain the essential amino acids necessary for humans in adequate amounts.[11] However, the idea that plant-based foods must be combined to provide humans all of the essential amino acids is outdated. The official position of the Academy of Nutrition and Dietetics is that protein from a variety of plant foods eaten during the course of a day supplies enough of all essential amino acids when caloric requirements are met.[12]

Protein per calorie

Protein content in foods is often measured in protein per serving rather than protein per calorie. For instance, the USDA lists 6 grams of protein per large whole egg (a 50-gram serving) rather than 84 mg of protein per calorie (71 calories total).[13] For comparison, there are 3 grams of protein in a serving of raw broccoli (91 grams) or 96 mg of protein per calorie (31 calories total). An egg contains twice as much protein per serving, but 12 mg less protein per calorie. The ratio of essential amino acids (the quality of protein) is not taken into account. It can be shown that common vegetable sources contain adequate protein, often more protein per calorie than the standard reference, whole raw egg, while other plant sources, particularly fruits contain less. It is recommended that adult humans obtain 10–35% of their calories as protein, or roughly 11–39 mg of protein per cal per day (22–78 g for 2000 cal).[14] A carrot has 23 mg protein per cal or twice the minimum recommendation, a banana meets the minimum, and an apple is below recommendation.[15]

Complete proteins in non-human animals

Scientists had known since the early 20th century that rats could not survive on a diet whose only protein source was zein, which comes from maize (corn), but recovered if they were fed casein from cow’s milk. This led William Cumming Rose to the discovery of the essential amino acid threonine.[16] Through manipulation of rodent diets, Rose was able to show that ten amino acids are essential for rats: lysine, tryptophan, histidine, phenylalanine, leucine, isoleucine, methionine, valine, and arginine, in addition to threonine. Rose’s later work showed that eight amino acids are essential for adult human beings, with histidine also being essential for infants. Longer term studies established histidine as also essential for adult humans.[17]

Interchangeability

The distinction between essential and non-essential amino acids is somewhat unclear, as some amino acids can be produced from others. The sulfur-containing amino acids, methionine and homocysteine, can be converted into each other but neither can be synthesized de novo in humans. Likewise, cysteine can be made from homocysteine but cannot be synthesized on its own. So, for convenience, sulfur-containing amino acids are sometimes considered a single pool of nutritionally equivalent amino acids as are the aromatic amino acid pair, phenylalanine and tyrosine. Likewise arginine, ornithine, and citrulline, which are interconvertible by the urea cycle, are considered a single group.[citation needed]

Effects of deficiency

If one of the essential amino acids is less than needed for an individual the utilization of other amino acids will be hindered and thus protein synthesis will be less than what it usually is, even in the presence of adequate total nitrogen intake.[2]

Protein deficiency has been shown to affect all of the body’s organs and many of its systems, including the brain and brain function of infants and young children; the immune system, thus elevating risk of infection; gut mucosal function and permeability, which affects absorption and vulnerability to systemic disease; and kidney function.[2] The physical signs of protein deficiency include edema, failure to thrive in infants and children, poor musculature, dull skin, and thin and fragile hair. Biochemical changes reflecting protein deficiency include low serum albumin and low serum transferrin.[2]

The amino acids that are essential in the human diet were established in a series of experiments led by William Cumming Rose. The experiments involved elemental diets to healthy male graduate students. These diets consisted of cornstarch, sucrose, butterfat without protein, corn oil, inorganic salts, the known vitamins, a large brown “candy” made of liver extract flavored with peppermint oil (to supply any unknown vitamins), and mixtures of highly purified individual amino acids. The main outcome measure was nitrogen balance. Rose noted that the symptoms of nervousness, exhaustion, and dizziness were encountered to a greater or lesser extent whenever human subjects were deprived of an essential amino acid.[18]

Essential amino acid deficiency should be distinguished from protein-energy malnutrition, which can manifest as marasmus or kwashiorkor. Kwashiorkor was once attributed to pure protein deficiency in individuals who were consuming enough calories (“sugar baby syndrome”). However, this theory has been challenged by the finding that there is no difference in the diets of children developing marasmus as opposed to kwashiorkor.[19] Still, for instance in Dietary Reference Intakes (DRI) maintained by the USDA, lack of one or more of the essential amino acids is described as protein-energy malnutrition.[2]

^Essential Amino Acids. phy-astr.gsu.edu: “Tillery points out that a number of popular ethnic foods involve such a combination so that in a single dish, one might hope to get the ten essential amino acids. Mexican corn and beans, Japanese rice and soybeans, and Cajun red beans and rice are examples of such fortuitous combinations.”